1. Field of the Invention
The present invention relates to a fitting device adapted to carry out a fitting process wherein, for example, a workpiece is inserted into a hole of an object.
2. Description of the Related Art
When a fitting process is carried out by using a fitting device having a robot and controlling force of the robot, an error in the orientation of a workpiece may be corrected by a procedure as shown in FIGS. 7 and 8. Concretely, as shown of part (a) of FIG. 8, for example, a cylindrical workpiece W2 is gripped by a robot and moved in a fitting direction D1, so as to fit W2 into a workpiece W1 having a fitting hole H1 corresponding to W2. First, after the workpieces contact each other, moment M due to the difference between the orientations of the workpieces is detected (step S101 of FIG. 7 and part (b) of FIG. 8). Then, the orientation of workpiece W2 is corrected due to the force control for the robot, so that moment M is reduced or close to zero (step S102 of FIG. 7 and part (c) of FIG. 8). Next, it is judged whether workpiece W2 is inserted into hole H1 of workpiece W1 by a predetermined length (step S103 of FIG. 7). If not, the procedure is returned to step S102. On the other hand, when workpiece W2 is inserted into hole H1 of workpiece W1 by the predetermined length (part (d) of FIG. 8), the fitting process is judged to be finished and the procedure is terminated (step S104 of FIG. 7).
Although the example of FIGS. 7 and 8 relates to the correction of the error in the orientation, the same is also applicable to the correction of an error in the position of the workpiece. In controlling the robot, command speed and command angular speed are calculated by multiplying the difference between a force or a moment applied to the both workpieces and a target force or a target moment by a parameter (force control gain). The detection of the force or moment, and calculation of the command speed and the command angular speed are performed with respect to each control period.
In addition to the above technique, Japanese Unexamined Patent Publication (Kokai) No. 8-168927 discloses a technique of detecting contact between a gripped workpiece and another workpiece into which the gripped workpiece should be inserted, and then reducing a gripping force against the workpiece. Then, a gripping part is moved in one plane so as to search a precise position of a hole and correct the position of the gripped workpiece, and the gripped workpiece is inserted into the hole after the gripping force is adjusted. Since the position of the hole is searched after the detection of the contact, the position of the hole may be precisely searched even when the position of the gripped workpiece is considerably different from the hole.
Japanese Unexamined Patent Publication (Kokai) No. 2004-167651 discloses a technique of searching an orientation component of a workpiece as well as a translation component. Even when it is difficult to detect a moment due to a large deviation of the orientation, a proper orientation of the workpiece may be searched by changing the orientation.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2008-264910 discloses a robot control system adapted to add a vibration force, wherein the magnitude and direction thereof periodically vary, to a workpiece to be fitted, so as to avoid the jamming of the workpiece during the fitting process.
In a control method of the prior art, since a moment generated by an error in the orientation of a workpiece is relatively small, it is difficult to correct the orientation based on the moment. For example, as shown in part (b) of FIG. 8, moment M due to the error of the orientation of workpiece W2 is generated by a resultant force of forces F1 and F2 applied to workpiece W2. In this case, since the directions of F1 and F2 are appropriately opposed to each other, the resultant force is smaller than F1 or F2, and thus the moment generated by the resultant force is relatively small. Therefore, such a small moment may be insufficient to correct the orientation of the workpiece. In particular, the rigidity of the robot or the workpiece is low, it is difficult to detect the moment required to correct the orientation of the workpiece. On the other hand, it is possible to increase the moment by increasing the pressing force between the workpieces; however, the possibility of damage to the workpiece increases.
Japanese Unexamined Patent Publication (Kokai) No. 2004-167651 discloses one solution for the above problem. FIGS. 9a to 9d disclose an example of a searching operation disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2004-167651. Concretely, FIGS. 9a to 9d indicate a position in a fitting direction (or the movement direction of the robot), a force applied to the workpiece in the fitting direction (or the pressing direction of the workpiece), an orientation of the robot about an axis, and a time variation of a detected moment, respectively. For simplification, only one component of the orientation is studied in this case. As shown in FIG. 9c, the orientation of the robot is repeatedly changed while the pressing operation. When the orientation is proper (in part L1 of FIG. 9a), the fitting of the workpiece progresses, on the other hand, when the orientation is not proper (in part L2 of FIG. 9a), the fitting of the workpiece does not progress. Accordingly, a technique is desired, by which the fitting process may be carried out in a short time and the frequency of the periodic application of the force as in FIG. 9b to the workpiece is reduced.
Similarly, in the technique of Japanese Unexamined Patent Publication (Kokai) No. 2008-264910, merely the reciprocating motion is carried out. Therefore, the fitting process may take a long time and excess force may be applied to the workpiece.